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The exact position of the secondary is not important. As long as you can see the entire primary once you're done, it's fine. What is really important is the orientation of the two mirrors.
Centring the reflected image of the spider may not be sufficient for satisfactory collimation. The spider may be slightly (few mm) off centre and it usually cannot be centred without drilling new mount holes in the OTA or bending the spider vanes. First centre the reflection of the centre spot of the primary (line up with X-hairs in sight-tube and/or use laser), then centre the reflection of the Cheshire.
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